I need to use a single LED but it’s brighter than I want. Can I just drive it at a lower current?
Yes, the light produced will be a function of the drive current. The linearity will depend on the specific make of LED.
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Some extra info: most of these LED modules are intended to be driven from a constant current source. The device datasheet will specify the light output at various currents. This particular module’s datasheet lists output at 350mA and 750mA.
http://www.leddynamics.com/wp-content/uploads/2019/01/A007_A008_IndusStar_v2.1.pdf
Thanks. The XPE is way too bright for what I need. I’m lighting a small diorama. I’m thinking about using the Luxeon C, and even at 350mA it’s probably too bright so I’m considering driving it at a lower current. I can always get a different resistor …
Nothing inherently wrong with underdriving LEDs - that’s where they’re at their most efficient and longest-lasting. Vf (forward voltage) will be lower so the resistor value might need to be a tad higher to account for the additional voltage drop if you want to hit an exact drive current but if you’re well below maximum current and using an eyeball lux meter using the specified nominal Vf for calculations should suffice. Manufacturers will usually include a graph on the spec sheet so you can estimate (or project) output at a given current.
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Unfortunately I’m not sure how much luminous output I need.
I’m working off this chart from flashlightfinder.com.
I’m illuminating a small diorama (roughly a 10" cube). As a WAG, I suspect 50 lumens is approximately in the right neighborhood, but I won’t really know until I see it. Therein lies my dilemma.
The spec sheet says I should expect about 116 lumens on the 4000K white LED, at 350 ma. This is one of the (the?) dimmest one-up LEDs I could find and I’m attracted to the available reflector/holder configuration because I need it to look like sunlight inside my diorama.
Absolute back-of-the-napkin, blithely assuming linear performance…
50 lm / 116 lm = 0.43 0.43 * 350mA = 151mA
The specific resistor will depend on the power supply’s voltage and the LED’s Vf, but subtract Vf from supply voltage to determine the voltage across resistor then use Ohm’s law to determine appropriate resistance using target current.
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Do note that there are potential shifts in the color spectrum when under driving LED, and that the shift can be different if simply current limiting, or PWM dimming.
I don’t think you will get enough to care, but just something to be aware of:
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Use a POT.
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+1 for this.
If you don’t know the brightness you want in advance, the potentiometer will allow you to literally dial it in.
Speaking from experience, you’ll want a logarithmic or audio taper potentiometer since a linear taper will appear to have nearly all of its variability at the bottom of the range.
Make sure the resistor / pot you use can handle / dissipate the power you put through it.
P = I^2 x R
May or may not be a problem. Build / test ckt before incorporating into your project.
Note: your basic pot can not handle much current.
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Thanks, but too darn many choices for a trogolodyte like me to wade through …